Clas Blomberg - Physics of life-Elsevier Science (2007)

Chapter 14. Thermodynamics formalism and examples 125
We make a distinction between two types of solvents. At one side, we have solvents such
as water where the solvent molecules are strong electric dipoles, and which we call “polar”.
Other polar liquids are alcohols such as methanol and ethanol, and ammonia, as well as acidic
liquids. At the other side with quite different properties, we have non-polar solvents where
the molecules do not have electric dipole moments or when those are very weak. Examples
are hydrocarbons, pentane and hexane (in various forms, benzene, toluene and so on).
14C
Water: solubility
As said, the solution entropy is important also at very low concentrations. Besides that, the
solution energy contributes to the chemical potential of solution. Energy is lost when molecules
are transferred from an external source, a salt or any other substance, to be dissolved in
a liquid, then more substance can be dissolved as the limit concentration gets larger. This leads
to the rule that “like likes like”; similar substances or substances which interact in similar ways
are more easily mixed than those of different types. All this explains much of solution features
of salts which dissociate to ions in a solvent or substances that as water are strong electric
dipoles, such as ammonia or ethanol, all of which dissolves easily in water. We can also understand
this for organic, non-polar solvents such as hydrocarbons (e.g. pentane), cyclohexane,
acetone benzene and so on. They easily solve similar organic compounds, also fats and oils
while all these compounds that dissolve easily are almost insoluble in water. This can be
rather easily understood in this case—the energy lost when molecules from a salt go out in a
non-polar solvent is large and more important than the entropy gain, which is always there.
This looks clear for non-polar molecules; substances similar to the solvent and which
feel similar forces as in pure conditions dissolve easily, while molecules that are electrically
charged as ions or electric dipoles loose too much energy when dissolved. The latter fact is
compensated in water. The interactions with water molecule are still of an electric nature,
and the energy loss by dissolving is less relevant.
Organic non-polar substances are almost insoluble in water. At a first glimpse, this might
seem to be ascribed the same principle. They are different from the water molecules and may
then also loose too much energy.
But stop, here we have to think harder. These organic, non-polar molecules interact by
what we call “van der Waals forces” (see Sections 5B and 6B). These are forces that always
are there, also for substances that appear completely electric neutral, such as noble gases.
They are as everything else basically of electrostatic origin and they are an effect of quantum
mechanics—classical mechanics would not assign any kind of force (but the very weak
gravitational one) between two helium atoms. Hydrocarbon molecules such as methane
interact by such rather weak forces, and that is the reason why the boiling point of methane
with about the same molecular weight and size as water is quite low.
But when we admit this, we see that the picture of solution of methane in water is somewhat
more difficult than we just discussed. A methane molecule feels almost the same van
der Waals force from the surrounding water as from other methane molecules. A solution in
water would rather have made interactions more favourable because of the higher density
in water than in a methane gas. The change in energy when methane is dissolved in water
cannot be the reason for the very low solubility. We have to find an explanation somewhere
else. If it is not an effect of methane energies, then it must be an effect of water.